Smooth surface chiller crystallizer

ABSTRACT

A chiller equipped with a scraper for crystallizing and separating p-xylene in a mixture of C8 aromatic hydrocarbons including p-xylene by indirect cooling, in which the scraped surface exhibits a surface roughness of less than 12.5-S as indicated in the Japanese Industrial Standard (JIS) B 0601-1970, is disclosed.

United States Patent 11 1 [111 3,722,229

Kayahara et al. 1 Mar. 27, 1973 SMOOTH SURFACE CHILLER [56] References Cited CRYSTALLIZER UNITED STATES PATENTS [75] Inventors: Ikuta Kayahara; Yusuke Tanaka,

both f Osaka, Japan 2,259,024 10 1941 Cleveland ....165 /l33 x 3,364,691 1/1968 Cottle ..62/68 [73] Assigneez Maruzen Oil Co., Ltd., Osaka,

Japan Primary Examiner-William E. Wayner [22] Filed: Dec, 29, 1971 AttorneySughrue, Rothwell, Mion, Zinn & Macpeak [21] Appl. No.2 213,614 ABSTRACT [30] Foreign Applic'afiml- Priority Data A chiller equipped a scraper for crystallizing and separatlng p-xylene m a mixture of C aromat1c Dec. 29, 1970 Japan ..45/ 120791 hydrocarbons including p-xylene byindirect cooling, in which the scraped surface exhibits a surface [52] US. Cl. ..62/354, 62/123, 165/94, roughness f 1 h 12 5 as indicated i the 165/133 Japanese Industrial Standard .115 B 0601-1970, is [51 Int. Cl ..F28f 13/00, F281 17/00 disclosed- 4 [58] Field of Search ...165/l33, 94; 62/123, 354, 342

6 Claims, 3 Drawing Figures H 0 5 100 Z 2 o X A o x A t: O :5: 8) 600 b Q Q E" 500- X o '2 0 E g 400- E x 300- x A X x 200 SCALE OF SURFACE ROUGHNESS PATENTEDMRZYIQYS 3,722,229

HEAT TRANSFER COEFFICIENT 7 H33 325 6-33 I253 553 50S IOOS- 2005 SCALE OF SURFACE ROUGHNESS BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a chiller equipped with a scraper in which the scraped surface has a surface roughness smoother than that of l2.5-S as indicated in the Japanese Industrial Standard (JIS) B aromatic hydrocarbons between the double-tube type chiller composed of a double-tube type scraped surface chiller or a plurality of a double-tube type scraped surface chiller connected in series.

2. Description of the Prior Art Various types of chillers for crystallizing p-xylene from a mixture of C aromatic hydrocarbons have hitherto been proposed and among them there is a chiller of the type having an outside jacket for circulating a coolant, placing therein a mixture of C aromatic hydrocarbons, and having a scraper for scraping off the crystals of p-xylene deposited on the inside heatexchange surface. There is generally employed a double-tube type scraped surface chiller, a tank type chiller, and the like.

In such cooling type crystallizers, the mixture of C aromatic hydrocarbons in the chiller is cooled to a temperature lower than the crystallizing temperature of pxylene by circulating a coolant through the outside jacket, whereby the crystals of p-xylene are formed in the mixture of C aromatic hydrocarbons and also the p-xylene is crystallized on the inside heat-exchange surface of the chiller. It is difficult to completely scrape off the crystals of p-xylene crystallized on the inside heatexchange surface by means of a scraper and consequently the crystals of p-xylene remaining without being scraped off form a layer of p-xylene crystals of a considerable thickness on the scraped surface, which greatly reduces the heat transfer coefficient of the chiller.

Thus, in the operation of such a conventional chiller equipped with a scraper, it is attempted, in order to reduce the thickness of the layer of p-xylene crystals formed on the scraped surface and increasing the heattransfer coefficient or the heat-exchange coefficient, to increase the pressure of the scraper against the scraped surface, whereby the crystal layer of p-xylene is forcibly scraped off. However, in attempting such, the scraper is severely abraded and it is necessary to increase, extraordinarily, the power for driving the scraper. Thus, owing to such a mechanical restriction, it is impossible in such a conventional manner to scrape completely off the layer of p-xylene. Such undesirable difficulty becomes more severe as the temperature difference between the coolant and the mixture of C aromatic hydrocarbons is increased, which greatly reduces the efficiency of the chiller.

Also, when employing a double-tube type scraped surface chiller, a system is employed in which a plurality of such double-tube type scraped surface chillers is connected to holding tanks, alternately in series, and pxylene is crystallized in each double-tube type scraped surface chiller, and the crystals of'-p-xylene are grown in the holding tank. Alternatively, the cooling and growing operations for forming a slurry of p-xylene crystals in a double-tube type chiller by crystallizing the p-xylene in a mixtureof C aromatic hydrocarbons, while circulating a large amount of the mixture of C chiller and a holding tank and growing the p-xylene crystals in the holding tank by retaining the slurry in the holding tank for a proper period of time, are repeated successively to obtain large crystals of p-xylene. Such a conventional system will further be explained by referring to the embodiment shown in FIG. 1 of the accompanying drawings.

FIG. 1 shows a simple flow sheet of the embodiment of a conventional crystallizing system and it will be understood that in the practical system, various addition means are required.

However, in an earlier era when the technique for separating p-xylene in a mixture of C aromatic hydrocarbons was through crystallization, such a technique was employed in which the mixture was cooled without circulating the slurry of the p-xylene crystals through a holding tank and a scraped surface chiller or without disposing a holding tank. The crystals of p-xylene were immediately separated from the mixture as disclosed in Industrial and Engineering Chemistry," No. 6, 1098 (1955) and U. S. Pat. Nos. 2,651,665 and 2,688,045. However, it is quite difficult in such a method to obtain large crystals-of p-xylene, which makes the separation of the p-xylene crystals from the mother liquor difficult. Thus, the yield of pxylene in a so-called one through step is low and accordingly this method has not been employed. Thus, the most systems employed recently are such that the growth of the crystals of p-xylene is conducted by circulating a large amount of the slurrythrough holding tanks and chillers. However, because in such a method double-tube type chillers and holding tanks are required and, further, the operation requires a long period of time due to the retention period in the holding tank, there are numerous disadvantages, such as the requirement of complicated means and a long cooling period.

SUMMARY OF THE INVENTION These faults of conventional methods are overcome by the present invention. That is, the advantages of this invention in connection with the aforesaid faults by the conventional methods are as follows:

1. For crystallizing p-xylene from a mixture of C aromatic hydrocarbons on cooling, the crystallization can be accomplished without substantially increasing the power of driving the scraper with the passage of operation time.

2. In the case of crystallizing p-xylene from a mixture of C aromatic hydrocarbons on cooling, the crystals crystallized on the scraped surface can be smoothly scraped off. 7

3. P-xylene in the mixture of C aromatic hydrocarbons can be crystallized on cooling without being accompanied by a reduction in the heat-transfer coefficient during the cooling operation.

4. In the case of crystallizing p-xylene from a mixture of C aromatic hydrocarbons on cooling, the cooling operation can be finished in a shorter cooling period of time than that of conventional methods.

5. In the case of crystallizing p-xylene from the mixture of C aromatic hydrocarbons on cooling, the logarithmic mean-temperature difference between the coolant and the mixture of the hydrocarbons can be increased as compared with that of conventional methods.

6. P-xylene can be crystallized from the mixture of C aromatic hydrocarbons under cooling through simplified means as compared with those in the conventional method.

An object of the present invention is to provide a chiller capable of being operated profitably, in industry, for cooling and crystallizing p-xylene in a mixture of C aromatic hydrocarbons and having an excellent heat-transfer coefficient.

That is, according to the present invention, there is provided a chiller equipped with a scraper therein in which the surface roughness of the scraped surface is smoother than 12.5-S as indicated in the JIS B 0601.

By using the chiller having the scraped surface of the surface roughness defined in the present invention, a constant driving power for the scraper, a smooth scraping faculty, and a high heat-transfer coefficient can be maintained.

Furthermore, according to the present invention, there is provided a crystallizing apparatus comprising one double-tube type scraped surface chiller or a plurality of double-tube type scraped surface chillers, connected in series, said double-tube type chiller having an estimation pump at the inlet for a mixture of C aromatic hydrocarbons and said apparatus having a bypass connecting an outlet of said chiller and the inlet conduit of said pump.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view showing a crystallizing system composed of a combination of a double-tube type scraped surface chiller and a holding tank according to a conventional technique;

FIG. 2 is a schematic view showing an embodiment of the system employing the apparatus of this invention comprising double-tube type scraped surface chillers connected in series; and

FIG. 3 is a graph in which figures are based on the case in which the cooling operation in conducted by employing the double-tube type scraped surface chiller and showing the relation between the overall heattransfer coefficient and the scale of surface roughness by reference to the JIS B 0601.

DETAILED DESCRIPTION OF THE DRAWINGS In FIG. 1 there is illustrated an embodiment wherein two sets of a combination of a double-tube type scraped surface chiller and a holding tank are used in two steps.

That is, a feed material, a mixture of C aromatic hydrocarbons is supplied from a conduit 1 and introduced through a conduit 2 into a double-tube type chiller 3 in which the mixture is cooled by a coolant circulating through an inlet 4 and an outlet 5 and then withdrawn from the chiller through a conduit 6. Thereafter, the cooled mixture is introduced into a holding tank by means of a pump 7 and in the holding tank, the crystals of p-xylene are grown. A part of the slurry in the holding tank 9 is withdrawn through a conduit 10 by means of a suction pump 11 and circulated into the double-tube chiller 3 through the conduit 2. Another part of the slurry in the holding tank 9 is introduced into another double-tube type chiller l3 through a conduit 12, wherein the slurry is further cooled by a coolant circulating through an inlet 14 and an outlet 15. The slurry in the chiller 13, thus cooled, is withdrawn therefrom by means of a pump 17 through a conduit 16 and then introduced into a holding tank 19 through a conduit 18, wherein the crystals of p-xylene are further grown. A part of the slurry in the holding tank 19 is also withdrawn therefrom through a conduit 20 by means of a pump 21 and circulated into the double-tube type chiller through conduit 12. Another part of the slurry in the holding tank 19 is introduced into a centrifuge 23 through a conduit 22, from which the mother liquor is withdrawn through a conduit 24 and the crystals of p-xylene are withdrawn through a conduit 25.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention provides a chiller for crystal- I lizing p-xylene from a mixture of C aromatic hydrocarbons by cooling. That is, according to the present invention, there is provided a chiller having a passageway for a coolant at the outside thereof, a passageway for passing a mixture of C aromatic hydrocarbons including p-xylene, and a scraper for scraping the crystals of p-xylene, crystallized on the scraped surface, the improvement wherein the surface roughness of the scraped surface is less than 12.5-S by reference to the Japanese Industrial Standard (JIS) B 0601-1970.

The surface roughness indicated by Japanese Industrial Standard B 0601-1970 is the numericalevaluation indicated by reference to maximum height (Rmax), ten point median height (Rz), and centerline average height (Ra). The surface roughness of a machined surface is assessed as a mean value of Rmax, Rz or Ra obtained from measurements taken at several places on the surface, which are selected at random, but the surface roughness employed in the specification of this invention is indicated by the maximum height (Rmax) of the amplitude of the roughness of the surface.

The maximum height (Rmax) is obtained by indicating in terms of microns (p) the maximum height of each selected portion of a standard length defined in the Standard.

Hereinafter, the part of the Japanese Industrial Standard B 0601-1970 having relation to the present invention is described below:

Surface Roughness of Japanese Industrial Standard- (JlS) B 0601-1970:

1. Scope This standard specifies numerical evaluations of surface roughness which are indicated by maximum height (Rmax), ten point median height (Rz) and centerline average height (Ra).

2. Terminology For the purpose of standardization, the following terms and definitions shall be adopted.

1. Surface Roughness: Surface roughness of a machined surface is assessed as a mean value of Rmax, Rz, or Ra obtained from measurements taken at several places on the surface, which are selected at random.

Remarks: 1. Since machined surfaces are not generally uniform, the values of surface roughness obtained from measurements at several places on a machined surface are not the same, but exhibit a certain amount of fluctuation. Therefore, measuring positions and numbers must be selected so that the mean value of the population of the surface roughness of the test surface can be effectively estimated in order to ascertain the surface roughness of a machined surface accurately.

. The surface roughness may be indicated by a value obtained from one measurement at a place on a machined surface, if the result satisfies the purpose.

2. Profile Curve: A profile curve shall be defined as a contour of a surface cut by a plane perpendicular to the mean surface of the surface to be measured.

Remarks: 1. Unless otherwise specified, the surface should be cut in such a direction as shown by the profile curve with respect to the largest value of surface roughness.

For example, where a surface to be measured has a predominant machined direction, the direction of the cross section shall be taken at a right angle to the lay.

2. When the profile curve is measured by a tracer method, the tip radius of the stylus should be smaller than 12.5 microns.

3. When a skid is employed, its radian in the direction of traverse shall be sufficiently large compared with the tip radius of the stylus. In the case when a modification of the profile curve (due to the skid) should be discussed, distance between the skid and the stylus, and radius of the skid shall be indicated clearly.

3. Sampling Length of Profile Curve: Maximum height and ten point median height is measured from a profile curve of a definite length. The definite length is termed the sampling length of the profile curve.

4. Roughness Curve: A profile curve which is obtained by an instrument whose frequency characteristic has a low frequency cut-off is termed a roughness curve.

5. Cut-off Value: A roughness curve shall be obtained by using a high pass filter in which a straight part of the attenuation curve has a maximum slope of 12 dB per octave. The cut-off value of the roughness curve shall be designated by the wavelength corresponding to the frequency assessed at 70 percent of the maximum transmission.

6. Mean Line of a Profile Curve or of a Roughness Curve: The mean line of a profile curve or of a roughness curve is defined by a line having the form of the nominal profile so that, within a selected part of the profile curve or the roughness curve, the sum of the squares of distances between the profile curve or the roughness curve and the mean line is minimum.

7. Centerline of Roughness Curve: The centerline of a roughness curve is defined as a line parallel to the mean line of the roughness curve, such that the surns of the areas contained between it and those parts of the roughness curve which lie on each side of it are equal.

3. Maximum Height 3.1 Definition The maximum height of a sampled profile curve is given by a distance expressed in micrometers .1.:

0.001 mm) measured in the direction of the vertical magnification of the profile curve, between two lines FIG. 1. Determination of the Maximum Height from the Selected Parts of a Profile Curve "Jim. was. u; w I, M .1 KW WW mmwggm L L and L Sampling length Rmax 1, Rmax 2 and Rmax 3: The maximum heights obtained from the selected parts L L and L Remarks: 1. The maximum height of a machined surface, as described in 2.1, is given by a mean value of the maximum heights of selected parts of each profile curve obtained from measurements at vari ous places of a machined surface to be measured.

. When a curved surface is measured, the maximum height should be obtained from measurement along a curve which must appear on the cross section.

3. The maximum height should be determined using profile curves without extraordinal peaks or valleys, which are considered as a fault.

3.2 Sampling Length The sampling length which shall be used for the evaluation of the maximum height of a selected part of the surface shall be, as a rule, chosen from the following six values: 0.08, 0.25, 0.8, 2.5, 8 and 25 (Unit:

3.3 Standard Values of Sampling Length Unless some other sampling length is specified, the

sampling length given in Table 1 shall be taken.

Table 1 Standard Values of Sampling Length for Measuring Maximum Height Range of maximum height Sampling length (mm) Up to 0.8 u Rmax 0.25 Over 0.8 u Rmax up to 6.3 p.

Rmax 0.8 Over 6.3 p. Rmax up to 25 y. Rmax 2.5 Over 25 p. Rmax up to p.

Rmax 8 Remarks: When the standard values of the sampling length given in Table l are used, there is no need to indicate the sampling length.

3.5 Preferred Index Values of Maximum Height When surface roughness is indicated by maximum height, the preferred index values given in Table 2 shall be used unless otherwise specified. These index values express the permissible largest values of maximum height. The symbol S shall be added to the index value of maximum height.

Table 2.

Values of S Series of Maximum Height (0.055) 0.88 12.58 SOS 2005 0.13 1.65 (188) (708) (2808) 0.28 3.25 258 100$ 4005 6.38 (358) (140$) (5605) As is clear from the table shown in the above ex planation of the Japanese Industrial Standard, the smaller the numerical value of the surface roughness, the smoother the surface.

For example, a surface roughness. of 12.5-S by the Japanese Industrial Standard shows that the height from the bottom of the deepest concavity to the top of the highest portion in the standard value of the sampling length being 2.5 mm is 12.5 microns.

Types of chillers used in this invention are a tubular type and a tank type chiller, which have a passageway for circulating a coolant at the outside thereof, a scraper for scraping off the crystals of p-xylene crystallized on the scraped surface in the chiller, said scraper being rotated via a driving shaft, and a passageway for passing a mixture of C aromatic hydrocarbons. For example, not only is a double-tube type scraped surface chiller and a tank type chiller applicable, but also any conventionally known chiller such as a screw type chiller, etc., may be applicable to this invention if they are provided with the above-mentioned features.

In conventional chillers, however, the surface roughness of the scraped surface is about 50-8 to about 100-S by the indication of JIS B 0601 and the overall heat-transfer coefficient for crystallizing p-xylene from a mixture of C aromatic hydrocarbons in an ordinary operation is about 200 to about 300 kcal/m-hr-C, but in the present invention the surface roughness is less than 12.5-S (including 12.5-S), which is far smaller than that of the conventional chillers. Furthermore, the

overall heat-transfer coefficient in an ordinary opera-- tion is about 660 kcal/m=-hr-C.

When pxylene is crystallized by cooling from a mixture of C aromatic hydrocarbons using a double-tube type scraped surface chiller, the chillers to be employed in this invention, one double-tube type scraped surface chiller or a plurality of double-tube type scraped surface chillers, connected in series, may be employed without disposing a holding tank connecting therewith the double-tube type scraped surface chillers asin the conventional manner.

In this case, a preferable crystallizing system is obtained by providing, if necessary, an estimation pump to the inlet of the double-tube type scraped surface chiller for introducing a mixture of C aromatic hydrocarbons and also providing a bypass connecting the outlet of the chiller for withdrawing a slurry of the mixture and the inlet conduit of the estimation pump to the system. The number of the double-tube type scraped surface chiller employed may be one, if it has a length sufficient for providing a desired heat transfer area, but because an ordinary chiller has an insufficient heat transfer area, the desired heat transfer area'is obtained by connecting a plurality of the chillers in series in a desired length. I

The apparatus in which the double-tube type chillers are connected in series requires the surface roughness to be less than 12.5-S. When the surface roughness is not less than 12.5-S by the indication of Japanese Industrial Standard B 0601, it is-necessary, for obtaining the pure p-xylene required, that a holding tank and a chiller be connected in series and that the operations of cooling, retaining and growing be conducted while a large amount of slurry is being circulated. Accordingly, it is required in the crystallizing system composed of a plurality of chillers connected in series without disposing a holding tank between the chillers, that the surface roughness of the scraped surfaces of the chillers are not over 12.5-S by the indication of the Japanese Industrial Standard.

The mixture of C aromatic hydrocarbons in the present invention means a mixture containing p-xylene and other xylene isomers and/or a small proportion of benzene.

A preferred embodiment of the present invention' employing the double-tube type scraped surface chillers, connected in series, will be explained by referring to FIG. 2, wherein .two double-tube type scraped surface chillers, each having a scraped surface of less than 12.5-S in the surface roughness by JIS B 0601-1970, are connected in series.

A feed material, the mixture of C aromatic hydrocarbons, is supplied through a conduit 1 and introduced into an estimation pump 4 through a conduit 3 by means of a pump 2. A predetermined amount of the mixture is then introduced into a double-tube type chiller 6 through a conduit 4 continuously. The mixture is cooled in the chiller by the indirect heat-exchange with a coolant circulated through a conduit 7 and a conduit 8 and thus p-xylene in the mixture is crystallized in the mixture and on the scraped surface. The crystal of p-xylene formed on the scraped surface is scraped by the scraper of the crystallizer. Thus a slurry containing crystals of p-xylene is formed. The slurry is then introduced through a conduit 9 into an estimation pump 10, from which a predetermined amount of the slurry is continuously introduced into a subsequent double-tube type chiller 1 1. The slurry is further cooled in the double-tube type chiller 11 by the indirect heatexchange with a coolant circulating through a conduit 12 and a conduit 13, and in the chiller 11 the crystallization of p-xylene and the growth of the p-xylene crystals are further conducted. The slurry is sent to a holding tank 14 through a conduit 15. The slurry is then introduced from the holding tank to a first centrifuge 17 through a conduit 16. The p-xylene crystals separated in the first centrifuge 17 are withdrawn therefrom through a conduit 18, while the remaining mother liquor thus separated is withdrawn from a conduit 19.

The mother liquor withdrawn from the first centrifuge is subjected to an indirect heat-exchange with the feed material entering the conduit 1 and thereafter, if necessary, may be supplied to an isomerization reaction zone to convert the isomers into p-xylene.

If the p-xylene crystals withdrawn from the first centrifuge through the conduit 18 are required to have a higher purity, they are supplied to a melting tank 20, wherein the crystals are melted by means of a heating coil 21. The molten p-xylene is then supplied through a conduit 22 into a chiller 23, in which it is recrystallized by a coolant circulating through a coolant line 24. The slurry of the recrystallized p-xylene is then introduced through a conduit 25 into a second centrifuge 26, from which the pure p-xylene crystals are withdrawn through a conduit 27 as a product and the mother liquor is withdrawn through a conduit 28. The mother liquor withdrawn from the second centrifuge may be, if necessary, recycled to the feed material to be supplied into the conduit 1. In addition, the second chiller 23 may be the chiller of this invention or may be another known chiller. Furthermore, in the melting tank of the embodiment, the p-xylene crystals separated in the first centrifuge may be melted completely or may be partially melted. The means for melting the p-xylene crystals may be made by supplying heat from outside the system or by recycling a part of the mother liquor from the second centrifuge through the melting tank.

As the recrystallizing step of the p xylene crystals from the first centrifuge, other known steps may be employed besides the above-mentioned step. Also, in the abovementioned example, the double-tube type scraped surface chillers 6 and 11 are employed in the first crystallizing step, but other types of chillers may be employed as the chillers if the scraped surface has the surface roughness defined in the present invention.

In the present invention, by combining a bypass 29 or 30 with the estimation pump 4 or respectively, the capacity or the flow rate of the pump can be controlled. For example, when an estimation pump is set so that a predetermined amount of the mixture or the slurry may be supplied and the amount of the mixture or the slurry to be introduced into the estimation pump may be insufficient, a part of the mixture or slurry withdrawn from the chiller is recycled into the estimation pump through the bypass to supplement the insufficient amount of the mixture or the slurry. On the other hand, if an excessive amount of the feed material or the slurry is supplied to the estimation pump, a part of the materi- Hit al is passed through the bypass 29 or 30 and enters into the conduit 9 or 15, respectively, without being introduced into the chiller, whereby the amount of the feed material or the slurry can be controlled. Moreover, if the chiller or the estimation pump causes any trouble, the whole crystallizing system can be operated by utilizing the bypass by stopping the operation of the damaged device. The employment of the bypass is particularly effective when a plurality of chillers are connected in series and also the employment of the estimation pump is more effective as the length of the cooling'zone is longer. That is, if the cooling zone of the chiller is long, a pressure drop occurs near the outlet end to disturb the smooth flow of the material to be cooled and thus a pump capable of sending the material through the chiller with high pressure is required. However, when one pump having a high discharging power or pressure is employed in such case, there occurs a problem in regard to the pressure resistance of the material of the chiller near the inlet thereof and, furthermore, leakage of the liquid mixture through the ground of the shaft of the scraper equipped to the chiller occurs owing to the high pressure of the flowing liquid. However, by employing an estimation pump, such difficulties can be overcome.

In the embodiment shown in FIG. 2, the slurry withdrawn from the final double-tube type chiller is retained once in the holding tank before it is supplied to a separation step for being separated into the pxylene crystals and the mother liquor, but according to the dimensions of the heat-exchange area of the chiller and the conditions for cooling, the employment of the holding tank is not always necessary or the slurry may be directly introduced into the separation step. In the separation step, it is preferable to employ a mechanical means for separating the p-xylene crystals from the mother liquor, such as a centrifuge.

Also, the mixture of C aromatic hydrocarbons to be supplied to the chiller in this invention is usually in a liquid state, butit is preferable for the purpose of growing the crystals of p-xylene to incorporate preliminarily the crystals of p-xylene.

The coolant used in the apparatus of this invention may be conventional, such as a light hydrocarbon, e.g., ethylene or propylene, a carbon dioxide gas, freon, ammonia, and a liquid cooled by the aforesaid coolant.

Hereinafter, the examples of crystallizing by using the apparatus of this invention will be shown below together with comparative examples. However, the present invention is not deemed limited to these examples.

EXAMPLE 1 A mixture of C aromatic hydrocarbons including pxylene was introduced into a double-tube type scraped surface chiller equipped with a scraper to crystallize the p-xylene. The chiller is composed of one cell containing one pipe therein so that the pipe may be bent at the ends of the chiller to form seven turned passageways. The length of one passageway of the pipe was 10 m and the diameter of the pipe was 6 inches. A scraper is equipped in the pipe and the aforesaid mixture of hydrocarbons was passed through the pipe and cooled by passing propane as a coolant through the space between the outer wall of the pipe and the cell of the chiller. The cooling operation was conducted under the conditions that the temperature of the feed material at the inlet of the chiller was 20 C, and the temperature of the coolant was 4 C, -7 C, or 12" C. In the example, three kinds of the chillers were used, each of which having a scraped surface was divided into three zones, each of which had a surface roughness of less than 12.5-S by reference to JIS B 0601. Their overall heat-transfer coefficients were measured according to the operation conditions, the results of which are shown in FIG. 2 of the accompanying drawings.

In the graph of FIG. 2, the relation of the overall heat-transfer coefficient (kcal/m -hr-C) (ordinate) of the chiller and the surface roughness (abscissa) of the scraped surface of the chiller is shown. Each section of the abscissa shows the region of each divided section and is not a scale divided by the same interval. Also, the points (a), (b), and (c) stand for the cases where the temperature of the propane was 4 C, 7 C, and 12 C.

COMPARATIVE EXAMPLE 1 The same procedure as Example 1 was followed except that four kinds of chillers were used, each of which having a scraped surface divided into four zones, each of which having a surface roughness of larger than 12.5-S by reference to JIS B 0601. The results are shown in FIG. 2.

As shown in the graph of FIG. 1, it is clear that in the region of the scraped surface having a surface roughness over 12.5-S, the overall heat-transfer coefficient was lower as the temperature difference between the feed material and the coolant was larger, whereas in the regions of the scraped surface having a surface roughness of less than 12.5-S, the overall heat-transfer coefficient was about 660 kcal/m -hr-C regardless of the temperature difference between the feed material and the coolant as shown in the results of Example 1 described above. It will be clearly understood from the results of Example 1 and Comparative Example 1 that the overall heat-transfer coefficient is greatly changed with the surface roughness of 12.5-S as the critical value. ,Moreover, when the chiller having the surface roughness of less than 12.5-S wasemployed, no reduction in the overall heat-transfer coefficient with the passage of time was observed.

EXAMPLE 2 In this example, the mixture of C aromatic hydrocarbons having the composition as shown in the table below was used as the feed material and ethylene was used as the coolant.

Four double-tube type chillers were used in series, each of which having a scraped surface and having a surface roughness of 5-8 by reference to JIS B 0601 and containing a pipehaving a diameter of 6 inches. The pipe was bent at the ends of the chillers so that 14 tubular passageways might be formed and the length of one passageway was 11.3 m. In the separation step, a centrifugal separator was employedand one holding tank was provided between the final chiller and the centrifuge. In this example, only one pump was employed for supplying the feed material to the first chiller and no bypass was employed. The operation conditions and the results obtained are shown in the following table together with those of the following comparative example.

COMPARATIVE EXAMPLE- 2 The same feed material and coolant as in Example 2 were used. The double-tube type chiller used had a scraped surface having a surface roughness of -8 on the average by reference to JIS B 0601 and contained a pipe having a diameter of 6 inches. The pipe was bent at the ends of the chiller so that 16 passageways were formed and the length of one passageway of the pipe was 11.3 m. Two sets of the combinations, each composed of the double-tube type chiller and a holding tank, were connected with each other in series as shown in FIG. 1 of the accompanying drawings. The slurry was circulated between the holding tank and the chiller to conduct repeatedly the operations of crystallization and the growing of the crystals. Four pumps were provided to the inlets and outlets of the two holding tanks and as a separation step, a centrifuge was employed. The cooling conditions and the results are shown in the following table together with the cooling conditions and the results in Example 2:

Example Comparative Example Heat-exchange area 157 m 411 in Heat absorbed by coolant 1.76 X 10 kcal/hr 1.92 X 10 kcal/hr aromatic hydrocarbons other than p-xylene The At value in the above table is the logarithmic mean temperature difference between the feed material and the coolant calculated from the difference between each temperature of the feed material at the inlet and outlet of the chiller or the inlet and the outlet of the holding tank and each temperature of the coolant at the inlet and the outlet of the chiller.

The purity of p-xylene obtained from the first centrifuge was 87.2 percent in Example 2 and the same in Comparative Example 2.

In addition, as is clear from the above table, in the crystallizing syste-m according to the present invention, the heat-exchange area of the chiller may be smaller than that in the conventional system and also the amount of the coolant and the retention period of time are less than those in the conventional system. Also, a clear difference was observed with respect to the At value defining the necessary heat-exchange area.

EXAMPLE 3 An estimation pump was provided in front of each of the two chillers in Example 2 and a bypass connecting the outlet of the chiller and the inlet of the estimation pump was formed to each combination of the chiller and the pump. The estimation pump had a flowing capacity of 80 tons/hr and tons/hr of the feed material was supplied to the chiller. Thus, 20 tons/hr of the excessive feed material was passed through the bypass in the same direction as in the chiller and accordingly the amount of the feed material supplied through the estimation pump into the chiller was controlled to 80 tons/hr. In this case, no reduction in pressure was observed in the outlet of the chiller. The purity of p-xylene recovered from the first centrifuge was 86.5 percent and the results obtained were almost the same as the results of Example 2 shown in the above table.

EXAMPLE 4 By using the same apparatus as in Example 3, 70 tons/hr of feed material was supplied. Thus, after a definite period of time, tons/hr of the deficient feed material was passed through the bypass to a direction opposite to the direction of the feed-material in the chiller to control the amount of the feed material supplied through the estimation pump into the chiller to 80' tons/hr. In this case, no reduction in pressure was observed in the outlet of the chiller. The purity of pxylene recovered from the first centrifuge was 87.1 percent and the results obtained were the same as those of Example 2 shown in the above-mentioned table.

As mentioned above, the separation efficiency of pxylene from the mixture of C aromatic hydrocarbons by the chiller of this invention was not inferior to that of a conventional method employing a method of circulating a large amount of slurry. Furthermore, since in the conventional method the supersaturation degree of the slurry is gradually increased as the slurry approaches from the inlet to the outlet of the chiller and crystals of p-xylene are formed on portions other than the scraped surface of the chiller, which reduces the smooth rotation of the scraper and causes a clogging in the tube, a cooling method of circulating the slurry is employed by combining the chiller and a holding tank for promoting the growth of the crystals. Accordingly, the conventional problems normally present can be completely removed by the present invention.

Moreover, as is clear from the above description, according to the present invention, the crystals of pxylene crystallized on the scraped surface of the chiller can be smoothly scraped off with no necessity of increasing the power for driving the scraper with the passage of time, as is the case of crystallizing by cooling the mixture of C aromatic hydrocarbons including i)- xylene in the chiller. Thus, according to the present invention, the layer of the p-xylene crystals is not left on the scraped surface of the chiller, and therefore the heat-transfer coefficient of the chiller itself is not reduced with the passage of time. Therefore, the chiller of this invention is also superior to conventional chillers in the aspect of the heat-transfer coefficient.

Further features of this invention include the fact that the apparatus can be simplified by the omission of the circulation of the slurry by using a holding tank and that the cooling period of time can be quite reduced by the saving of the circulation in the holding tank and the retention period of time in the holding tank.

Furthermore, because the At value in the apparatus of the present invention is larger than that in the technique of an earlier era when a technique of separating p-xylene by crystallization was disclosed and that of the method emplo ing a manner for circulating a large amount of slurry y using a holding tank, the heat-exchange area in the present invention may be smaller and thus the present invention is quite excellent in cooling efficiency.

Although the present invention has been illustrated in detail by way of applicants specification and examples included therein, it is readily apparent that various changes and modifications can be made without departing from the spirit and scope thereof.

What is claimed is:

1. In a chiller having a passageway for a coolant at the outside thereof, a passageway for'passing a mixture of C aromatic hydrocarbons including para-xylene and a scraper for scraping the crystals of para-xylene crystallized on the scraped surface thereof, the improvement wherein:

the surface roughness of the scraped surface is less than l2.5-S by reference to the Japanese Industrial Standard B 0601-1970.

2. The chiller of claim 1, wherein said chiller is a double-tube type scraped surface chiller.

3. The chiller of claim 1, wherein said chiller is a tank type chiller.

4. The chiller of claim 2, wherein one or more of said double-tube type scraped type chillers are employed.

5. The chiller of claim 4, wherein an estimation pump is equipped at the inlet side of each of the chillers for' introducing the mixture of C aromatic hydrocarbons, including para-xylene, to the chillers.

6. The chiller of claim 5, wherein a bypass is formed between the outlet of each chiller for the mixture of C aromatic hydrocarbons and the inlet of the estimation pump. 

2. The chiller of claim 1, wherein said chiller is a double-tube type scraped surface chiller.
 3. The chiller of claim 1, wherein said chiller is a tank type chiller.
 4. The chiller of claim 2, wherein one or more of said double-tube type scraped type chillers are employed.
 5. The chiller of claim 4, wherein an estimation pump is equipped at the inlet side of each of the chillers for introducing the mixture of C8 aromatic hydrocarbons, including para-xylene, to the chillers.
 6. The chiller of claim 5, wherein a bypass is formed between the outlet of each chiller for the mixture of C8 aromatic hydrocarbons and the inlet of the estimation pump. 